1. Field of the Invention
This invention relates to an improved variable delay memory system, which may be used as a digital delay line with a controllable, variable delay time. More particularly, it relates to such a memory system which may be implemented with a minimum number of functional elements. Most especially, it relates to such a memory system which may be incorporated in an ultrasonic imaging system and to an ultrasonic imaging system incorporating the memory system.
2. Description of the Prior Art
The use of ultrasonic sound waves in apparatus for the examination of solid objects is now a well known and comparatively well developed art. In such apparatus, an array of ultrasonic transducer elements is used to transmit ultrasonic waves into the object, and reflections of waves from within the object are used to define geometry and related characteristics of the object's interior. Such ultrasonic imaging apparatus has been found to be particularly useful in medical applications as a non-invasive diagnostic tool. The state of the art in such medical applications has been reviewed, for example, by Havlice and Taenzer, "Medical Ultrasonic Imaging," Proceedings of the IEEE, Volume 67, No. 4, Apr. 19, 1979, pages 620 to 641.
Proposals have been made in the prior art to utilize digital electronics in ultrasonic imaging apparatus. For example, Waag, R. C., P. P. K. Lee and R. Gramiak, "Digital Processing to Enhance Features of Ultrasound Images," Proc. Ultrasound Symp., IEEE CH 1120-55U, pages 163-167, J. DeKlerk and B. R. MacAvoy, eds. (1976); and Corl P. D., P. M. Grant and J. S. Kino, "A Digital Synthetic Focus Acoustic Image System for NDE," Proc. 1978 IEEE Ultrasound Symp., pages 263-268 (1978), both disclose the use of such electronics in such systems. However, the systems disclosed there are not suitable for real time correction of time delay errors resulting from inhomogeneities in tissue or other objects being analyzed, nor for the correction of images distorted due to multipath interference of ultrasonic waves generated by the apparatus.
U.S. Pat. No. 4,290,310 discloses an ultrasonic imaging system incorporating digital electronics, in which memories are utilized as delay lines for steering and focusing ultrasonic transducer arrays. However, the delay line memory systems disclosed there do not incorporate any provision for varying the time delay based on the ultrasonic signals received by the apparatus.
Thus, while the art of ultrasonic imaging is a well developed one, there remains a need for the further development of variable delay memory systems for use as delay lines in such imaging systems.